A receiver for ASK or PSK bandspread signals having a code phase-locked loop for recovering the frequency and phase of the predetermined bandspreading code and a carrier phase-locked loop for recovering the frequency and phase of the carrier, though not designed for the Global Positioning System (GPS), is disclosed in published German Patent DE-C1 20 48 056.
Receivers for bandspread signals must be designed to be capable of receiving signals even if the latter are extremely noisy or lost in noise. Where the signal is evaluated, i.e., after demodulation with the aid of the code used for bandspreading, the signal should therefore be as narrow-band as possible, in order to contain as little noise as possible. The narrower the bandwidth of a receiver, however, the more difficult it will be to find the desired signal in case of frequency deviations. The capture range of a phase-locked loop, for example, is directly proportional to its bandwidth.
Bandspread signals are frequently employed in military equipment or in satellite communication, where greater relative speeds of transmitter and receiver result in a Doppler shift which is clearly greater than the desired receiver bandwidth. Therefore, the receiver must first be tuned to the current frequency, preferably using an automatic search strategy. Because of the narrow bandwidth required (order of 1 Hz), however, this tuning must not be very fast and may even lag behind the change in frequency due to the Doppler effect.
In the GPS, a total of 18 satellites are to form a uniform network around the earth and transmit navigation signals at a common frequency, but with different codes. From the signals received from four of these satellites at a time, whose directions should be as different as possible, one can determine the common system time and the receiver position in three coordinates. The GPS is described in greater detail in an article by J. J. Spilker, jr., "GPS Signal Structure and Performance Characteristics", Navigation: Journal of the Institute of Navigation, Vol. 25, No. 2, Summer 1978, pages 121 to 146.
To receive four satellites, use can be made of four separate receivers, but it is also possible to use a single receiver which receives the four satellites in succession. In this case, however, the problem of the tuning time becomes even more critical. It can be slightly alleviated by using the respective last determined values or value extrapolated therefrom as preset values. When the receiver is turned on or leaves a dead spot, however, the problem arises again with full severity.